The present invention relates to a method for compensating poor response time, and in particular, to a method and an apparatus for inhibiting flicker resulting from the poor response time of a liquid crystal display.
In recent years, besides cathode ray tubes (CRTs), liquid crystal displays (LCDs) have come into widespread use as display devices for various types of image displays and monitors for units such as personal computers (PCs) and television sets. The LCDs can be made significantly smaller and lighter than CRTs. In addition, many improvements in the display performance of LCDs, including low geometric distortion as well as considerably high picture quality, have been achieved. For these reasons, the LCDs have gained the spotlight as a mainstream display device used in video equipment of the future.
However, because of the poor response characteristic of the liquid crystal itself, the LCDs has the potential problem of poor response time. That is, in a typical display device used in the industry, the display is refreshed at a frame rate of 60 frames per minute, or, every (1÷60=) 16.7 ms. On the other hand, the response time of liquid crystals used in many current LCDs required to change from black to white is 10 to 50 ms, typically 20 to 30 ms. This means that one frame time in the display is shorter than the response time of most liquid crystals. As a result, problems, such as the visual persistence of moving images and inability to keep up with fast-moving images, caused by the response delay of the LCDs have become obvious.
The term xe2x80x9cresponse timexe2x80x9d used in the industry refers to the sum of (1) time required to reverse color by applying a voltage to a liquid crystal cell and (2) time required to restore the original color by the removal of the applied voltage. The term xe2x80x9cframexe2x80x9d used in the industry represents the scanning of all the images (picture elements) that should form one complete picture on the display.
Some solutions to these poor response time problems with the LCD are disclosed in, for example, Published Unexamined Japanese Patent Applications Nos. 2-153687, 4-365094, 6-62355, and 7-56532.
In Published Unexamined Japanese Patent Application No. 2-153687, a LCD is provided which is configured to discriminate between a static image area having less motion and a fast-moving area and apply a signal process only to the moving area to emphasize time-based changes in an image, thereby improving response time in the image area where better response time is required to reduce visual persistence and noise.
In Published Unexamined Japanese Patent Application No. 4-365094, a LCD is provided which is configured to be driven by reading pre-stored optimum image data according to the direction and degree of a change when the image data changes, thereby allowing the LCD to rapidly follow the fast-changing image.
In Published Unexamined Japanese Patent Application No. 6-62355, a technology is disclosed which superposes a difference component between fields or frames on a video signal to provide pulse stepping drive when the video signal changes between the fields or frames, thereby improving the response of display elements in an LCD.
In Published Unexamined Japanese Patent Application No. 7-56532, a technology is disclosed which provides table memory containing a table of image increase/decrease values and drive a liquid crystal panel (liquid crystal cell) by performing an addition/subtraction in order to improve response changes due to changes in the gray scale in the liquid crystal panel. However, the amount to be added or subtracted is expressed only by the word xe2x80x9coptimumxe2x80x9d and no specific amount is disclosed.
One problem associated with picture quality in LCDs which do not arise in a CRT display is flicker. When, for example, a wire-frame model in a CAD application is displayed on the LCD and the operator (user) moves it continuously at a relatively low speed, about several tens pixels per minute, the entire wire-frame model appears to blink in a cycle of several to several tens Hz. This effect is called flicker. While this effect does not occur in CRT displays, it occurs in most existing types of LCDs and many customers have requested minimization of the flicker urgingly. The flicker herein differs in symptom and cause from that in CRT displays which is caused by infrequent refresh.
In CAD applications, a wire-frame model is typically displayed using many thin lines in white or other colors against a black background. Assuming that the wire-model is white (all of the colors R (read)/G (green)/B (blue) are xe2x80x9cONxe2x80x9d) as an example, no problem arises when the model stay stationary on the screen because only a few frames are required to achieve an proper brightness. However, if the operator move the model on the screen, the proper brightness cannot completely be achieved. That is, if a pixel is made light up only in one frame, the brightness of the pixel may not reach the predetermined brightness because the response of the LCD itself is slow as mentioned above. This situation will be described below with reference to the drawings.
FIG. 9 shows the movement of lines when a wire-frame model is moved on the screen. FIG. 10 shows on/off states of the pixels on line (i) in each frame at the time point in FIG. 9. FIG. 11 shows a change in the brightness of pixel (j).
Herein, as shown in FIG. 9, in the case where attention is paid to a particular pixel, assuming that a line of the wire frame 200 moves through frames (nxe2x88x921) 201 to (n) 202 to (n+1) 203 in sequence. That is, the pixel lights up in a time period equivalent to the frames in which the line passes over the pixel and goes off immediately after that.
Focusing attention on line (i) 205 represented by a dashed line, in particular, on the particular pixel, each frame is driven from OFF to ON by the movement of pixel (j) 206, then one frame after goes back from ON to OFF, as shown in FIG. 10. However, because the response time of commonly-used liquid crystals is longer than 16.7 ms, pixel (j) 206 changes back to black before completely returning from black to white. That is, as shown in FIG. 11, pixel (j) 206 is OFF in frame (nxe2x88x921) 201, goes ON in frame (n) 202, then goes OFF in frame (n+1) 203. However, the target brightness of pixel (j) 206 is not reached even though it is turned on in order to achieve 100% brightness in frame (n) 202. As a result, the brightness of the line drawing during movement will be low. The inventors have found that when a wire-frame model is continuously moved in a CAD application, the wire-frame model in fact repeatedly alternates between moving and stationary states every several frames and blinks due to a difference in display brightness between the moving and stationary states, and this difference causes xe2x80x9cflicker.xe2x80x9d
Many manufacturers have actively sought after a method for improving the response of LCD panels by improving a liquid crystal material itself or narrowing the gap between glass plates in order to reduce flicker of LCDs. Some state-of-the-art products on the market have an improved response time of about 25 ms including rising and falling time. Another LCD technologies for reducing response time to several ms have been disclosed in some academic conferences. However, these approaches to improve an LCD panel itself can hardly to provide mass-production products because of their low reliability, and there are many other problems to be solved to put them into practical use.
In view of these technical problems, it is a primal object of the present invention to inhibit the flicker effect as visual perception by the panel driving circuitry which drives an LCD.
It is another object of the present invention to drive the LCD by applying an offset to a moving model without globally determining whether the model is moving or stationary.
To achieve above-mentioned objects, a feature of the present invention includes a liquid crystal display comprises an input for inputting a video signal from a host and a storage medium for storing the previous brightness level of the video signal input through the input. A determinator is provided for determining an output brightness level based on the previous brightness level stored in the storage medium and the next brightness level of the next video signal input to the input, so as to make the time integration quantity of a brightness change substantially equal to an ideal quantity of light in a stationary state with respect to the next brightness level. Further included are drivers for driving an image displaying liquid crystal cell based on the output brightness level determined by the determinator.
Another feature of the present invention includes a liquid crystal display characterized by comprising a driver for driving each of the pixels forming an image for each frame to a liquid crystal cell displaying the image, an input for inputting an moving-state video signal which changes from the off state to the on state on transition to a particular frame in the frames and returns to the off state after the particular frame is completed, and elements for setting an offset for making the quantity of light closer to the quantity of light in a stationary state in which the moving-state video signal is continuously turned on for the particular frame. The liquid crystal display further includes a generator for applying the offset set by the setting elements to the moving-state video signal input through the input means to generate an output video signal, and an output for outputting the output video signal generated by the generation means to the drive means. By configuring the apparatus in this way, a difference in brightness between a stationary state and a moving state which can be considered as the principal cause of flicker can be reduced to inhibit visually perceptible flicker.
Yet another feature of the present invention is further characterized by a liquid crystal control circuit having a function for inhibiting flicker caused by a difference in brightness when an input wire-frame model is displayed by liquid crystal cells. The liquid crystal control circuit includes a storage portion for storing an offset in brightness in a moving state in which the wire-frame model having a predetermined gray scale changes from frame to frame with respect to a particular pixel. This is with relation to brightness output in a stationary state in which the wire-frame model having the predetermined gray scale is displayed on the particular pixel across a plurality of frames. Further included is a correction portion for applying the offset stored in the storage portion to the gray scale of the wire-frame model if the input wire-frame model is in a moving state.